The extraction of intravenously administered rubidium-86 in C3H mouse kidneys 4 mos after irradiation was used as a measure of renal blood flow. Seven fractionation schedules of 240-kV X rays were employed with over-all times of either 4 or 18 days, using 4, 5, 6, 9, or 15 equal dose fractions. The reduction in86 Rb uptake was found to be similar after 2000 rets given as 5 fractions in 4 days (5F/4d) or 9F/4d as after 1900 rads given as a single dose. For equal ret doses administered in 5 or 9 fractions (24-hr or 12-hr intervals, respectively) over 4 days, the reduction in86 Rb uptake was almost identical. After 4, 6, 9, and 15 fractions in 18 days, however, a significantly smaller effect was found than in the 4-day schedules for similar ret doses. No increase in the thymidine labeling index of the tubule cells in irradiated kidneys was seen. These results indicate that fractionated irradiation reduces perfusion to the kidneys in a predictable fashion. They also suggest that, for this system, the fraction exponent in the Ellis formula appears to be valid, but the time exponent of the formula was not confirmed. Possible explanations for this finding are discussed.

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